There are many applications where it is necessary to provide a relatively small, low-loss transmission line structure for radio frequency signals. One such application is in modern communications systems, where it is desirable to provide a radio transceiver which packs higher performance and greater efficiency into a package having smaller size and lighter weight.
Transmission line structures, such as resonators or filters, can be formed on dielectric substrates. For example, conventional stripline or microstrip resonators typically utilize a substrate which can be a ceramic or another dielectric material. For microstrip construction a metallized runner comprising one or more resonators or conductors is formed on one side of the substrate with a ground plane on the other side. The stripline configuration utilizes two such structures with ground planes on the outside and the runner therebetween.
Although the stripline resonator structure described above performs acceptably as a resonator, current bunching occurs at the cross-sectional corners of the conductor runner located between the two dielectric substrates. This non-uniform current density or current bunching results from sharpness of the corners of the runner. Ideally, for uniform current density, the conductor should be cylindrical as in some block filters. Because of the sharp corners, the resultant non-uniform current density of the conductor effectively increases the resistance exhibited by the resonator. It is well known that such increases in resonator resistance correspondingly degrades the quality factor or Q of the resonator.
For purposes of this document, Q.sub.U is defined as the unloaded quality factor of a particular resonator which is uncoupled to any adjacent resonators. Q.sub.L is defined as the loaded quality factor of a particular resonator which is coupled to a resistive source or load. The ratio Q.sub.L /Q.sub.U of adjacent or edge coupled resonators determines the passband insertion loss of a stripline filter which employs such resonators. Thus resonators with a low QL/QU ratio result in filters with low insertion loss. That is, the higher unloaded Q or Q.sub.U for a given Q.sub.L, then the lower is the insertion loss of the stripline resonator filter. Hence, non-uniform current distribution in resonators result in higher resistance which also results in lower unloaded Q or higher insertion loss.
To combat current bunching at the resonator corners, one prior art method provided an elliptically shaped resonator structure by locating the center resonators or runners in grooves that were elliptical or at least substantially rectangularly shaped with rounded corners to approach the ideal "smooth" circular shape. However, in manufacturing, the structure of ceramic substrates does not lend itself easily to a groove having rounded corners.
In addition, since the groove increases the effective thickness (t) of the conductor as compared to a thin metallized layer conventionally deposited on top of the dielectric, the thickness of the dielectric (b) also had to be increased to maintain an optimum t/b ratio. Hence, the overall size of the stripline will correspondingly increase in height. It is a well established relationship or ratio that for a certain cross-sectional thickness "t" of the center conductor, there is a distance "b" between the opposing ground planes of the stripline that is required for an optimum unloaded Q or Q.sub.L to provide an optimum characteristic impedance and a resultant low insertion loss. However, as more dielectric material is needed to grow the stripline in height, the more expensive the stripline becomes.
Another major problem with microstrip filters in the past has been in coupling the individual edge coupled resonators. In conventional microstrip transmission lines, the amount of coupling between adjacent resonators is limited to how close the lines are capable of being deposited. Electrical coupling between the edge coupled conductive strips or resonator runners is achieved by means of fringing electromagnetic fields associated with each conductive strip or resonator. The fringing electromagnetic field of a single strip affects adjacent strips to a degree dependent upon the physical distance between the two adjacent strips. Increased coupling is desired since as the coupling is increased, the bandwidth of the filter also increases as the selectivity, Q, and insertion decrease. Thus a wider bandwidth also reduces the insertion loss of the filter.
Hence, a low cost and miniature microstrip or stripline resonator that provides increased coupling or optimum characteristic impedance while keeping insertion loss relatively low is desired.